A digital hearing aid device typically comprises an input transducer such as a micro-phone to receive sound from an ambient environment and convert the received acoustic signal to an electrical audio signal. The electrical audio input signal is a signal in the frequency domain and is converted to a time-domain input signal using an analog-to-digital converter. Further, the time-domain input signal is converted to a number of input frequency bands. Typically, the number of input frequency bands is determined by an analysis filter bank which also includes filter coefficients to provide gain to selected frequency bands, e.g. according to a specific hearing situation. In a processing unit the number of frequency bands is processed in a number of processing channels. The processing of the number of frequency bands requires sufficient computational power and consequently energy provided e.g. by a battery. Eventually, the processed frequency bands are converted via a digital-to-analog converter into an electric audio output signal that in turn is converted into audible sound and emitted as an acoustic output signal into an ear of a user using a (loud-)speaker, also called a receiver. The speaker can be located in the ear canal of the user. Based on a hearing aid device as described above the hearing experience of a user can be improved.
Typically, a digital hearing aid device can be programmed by connecting it to an external computer. This allows additional processing features to be implemented. Thereby, the processing characteristics can be adjusted enabling e.g. an amplification or suppression of selected frequency bands. Sometimes processing characteristics can be adjusted by a user itself according to different hearing situations. Even programs that adjust the processing characteristics automatically and adaptively can be implemented. Based on such programs e.g. acoustic feedback or background noise can be adaptively reduced or the processing of the received sound signal can be automatically adapted to different hearing situations. Consequently, a user's comfort can be increased.
In order to decrease the computational effort and, thus, to save energy, it can be advantageous to bundle input frequency bands and to allocate a smaller number of frequency bands to be processed to processing channels of a signal processing unit. After processing the smaller number of frequency bands, the processed frequency bands can be redistributed to a larger number of output frequency bands such that the resolution of frequency bands is increased again.
EP 3122072 A1 describes an audio processing device comprising an input unit for converting a time-domain input signal to a number of input frequency bands and an output unit for converting a number of output frequency bands to a time-domain output signal. The object of the described audio processing device is to provide a flexible audio processing scheme, e.g. adapted to characteristics of the input signal. This allows that the audio processing can be adapted to a particular acoustic environment and/or to a user's needs (e.g. hearing impairment) with a view to minimizing power consumption and/or processing frequency resolution.
Hearing aid devices can be implemented e.g. as behind-the-ear (BTE) hearing aids which typically have the microphone arranged behind the ear canal of a user or as in-the-ear (ITE) hearing aids which typically have the microphone arranged in the ear of a user. In both cases a speaker is typically placed inside a user's ear canal in order to stimulate the eardrum. An advantage of a BTE hearing aid is that the distance between microphone and speaker, also known as feedback path, is larger compared to an ITE hearing aid such that the BTE hearing aid is less affected by feedback. As a consequence, in BTE hearing aids a higher gain can be applied to individual frequency bands without resulting in feedback.
In case a hearing aid device, e.g. a BTE or an ITE hearing aid, comprises a directional microphone with two microphones or two sound inlets, the directional system can be configured such that the directional pattern aims at cancelling the feedback path. This means that the directional response has its minimum directivity towards the feedback path. The directional pattern represents the directionality of the directional system.
In U.S. Pat. No. 9,351,086 B2 an ITE hearing aid is described which inter alia comprises a directional microphone and a feedback suppression system for counteracting acoustic feedback on the basis of sound signals detected by the two microphones or the two sound inlets. The described hearing aid device comprises an “open fitting” providing ventilation. The two microphones or the two sound inlets of the directional microphone (forming part of a directional system) are arranged in the ear canal at the same side of the receiver and sound is allowed to propagate freely between the microphones or between the inlets of the directional microphone and the receiver. It is preferred that the hearing aid device comprises a procedure (such as an adaptive procedure) for optimizing the directional system of the hearing aid device. Thereby, an improved feedback reduction is achieved, while allowing a relatively large gain to be applied to the incoming signal.